A digital subscriber line (DSL) is a high-speed data transmission technology for transmission over a phone twisted pair, for example, an unshielded twisted pair (UTP). There are multiple DSL lines in a DSL system. Currently, a DSL access multiplexer (DSLAM) is generally used on a network side to provide access services for the multiple DSL lines. Due to an electromagnetic induction principle, mutual crosstalk occurs between signals on the multiple DSL lines that access the network side.
FIG. 1 is a schematic working diagram of synchronous sending and synchronous receiving in a DSL system. In FIG. 1, each user accesses a network side by using a twisted pair. A downstream and upstream shared channel H on the kth tone in a frequency domain may be expressed in a matrix form:
  H  =            [                                                  h              11                                                          h              12                                            …                                              h                              1                ⁢                                                                  ⁢                M                                                                                        h              21                                                          h              22                                            …                                              h                              2                ⁢                                                                  ⁢                M                                                                          ⋮                                ⋮                                ⋱                                ⋮                                                              h                              M                ⁢                                                                  ⁢                1                                                                        h                              M                ⁢                                                                  ⁢                2                                                          …                                              h              MM                                          ]              M      ×      M      
hij is a transmission equation of a pair j to a pair i. In an actual case, maximum values of i and j are equal, and are equal to a quantity of lines in the DSL system, and the quantity is assumed to be M. Therefore, H is expressed as an M×M channel transmission matrix. Further, it is assumed that x is an M×1 channel input vector, y is an M×1 channel output vector, and n is an M×1 noise vector. Therefore, an ultimate channel transmission equation is expressed in the following form:y=Hx+n 
Crosstalk of a twisted pair is strong in a high frequency. In an example, to eliminate crosstalk, a vectored DSL technology may be used to eliminate far-end crosstalk. In an existing vectored DSL technology, based on characteristics of performing joint sending and receiving on the network side, a signal processing method is used to cancel interference caused by far-end crosstalk FEXT. A premise of using the characteristics of performing joint sending and receiving on the network side is that symbols are synchronous. However, in a strong crosstalk scenario, a signal-to-noise ratio (SNR) of a signal received by CPE is excessively low. As a result, the customer-premises equipment CPE on a user side cannot precisely maintain synchronization with a network side device, and therefore, cannot precisely feed back a clipped error sample to a VCE. Consequently, FEXT elimination is affected.